The invention pertains to an exhaust gas system of an internal combustion engine, especially of a motor vehicle, with a NOx storage catalyst, where a cooling section is provided upstream of the catalyst.
For spark-ignition engines with direct fuel injection, NOx storage catalysts are being used increasingly as a way of converting nitrogen oxides. Because sufficiently high conversion rates are achieved with such catalysts only in relatively narrow exhaust gas temperature windows, it is necessary to coordinate the operation of the engine and the catalyst system with each other in such a way that the exhaust gas temperatures at the inlet to the storage catalyst are within the temperature window over the widest possible operating range.
To prevent the upper temperature limits from being exceeded, the use of exhaust gas systems with a cooling section is a reasonable approach, where this cooling section effectively lowers the temperature of the exhaust gas. Thus the thermal load on the catalyst system is also reduced simultaneously and the service life of the catalyst is prolonged. For example, an exhaust gas after-treatment device for a motor vehicle for the treatment of the exhaust gas stream with a three-way catalyst and a downstream NOx trap is known from DE 199 05 345 A1. Between the three-way catalyst and the NOx trap, the exhaust gas is conducted along a temperature-changing route with sensitive parallel paths with continuous flow. This has the result that fast-flowing exhaust gas flows primarily along a route with higher heat energy dissipation, whereas slow-flowing exhaust gas flows primarily along a route with lower heat energy dissipation, so that the temperature upstream of the NOx trap can be maintained within a predetermined window. As a result of this measure, the relative heat dissipation (amount of heat dissipated relative to the total amount of heat present in the exhaust gas) is reduced for low exhaust gas mass flow rates, whereas the relative heat dissipation is increased for high exhaust gas mass flow rates. This arrangement is complicated and expensive, however, especially when maintenance or repairs are required.